1. Field of the Invention
The field of the invention relates generally to variable displacement engines for providing improved fuel economy.
2. Background of the Invention
Variable displacement engines are known in which the intake valves of selected cylinders are mechanically disabled and the remaining cylinders continue to operate. This results in improved fuel economy because the remaining cylinders operate with a wider throttle angle than is the case when all cylinders are operating.
The inventors herein have recognized numerous problems with the prior approach to variable displacement engines. One particular problem is that complex mechanical devices are required to deactivate the intake valves. Another problem is that rough idle conditions may result when in the variable displacement mode, particularly for engines in which the ignition firing order is not limited to each engine bank such as a V-8 engine. It has therefore been common not to operate such engines in idle speed modes.
Another approach has been to disable injected fuel to a selected number of cylinders. Such a system is described in U.S. Pat. No. 6,023,929, as well as in JP-A-55 029002 and JP-A-55 59549.
However, the inventors herein have also recognized that such systems do not achieve the full benefit possible from such a system. In particular, the inventors herein have found, as described below, that engines can obtain further fuel efficiency benefits than those provided by the above methods.
The above disadvantages and problems of prior approaches are overcome by a method for operating an engine having multiple combustion chambers. In one particular aspect of the invention, a method is disclosed for operating an engine having multiple combustion chambers, comprising determining a desired engine output, when the desired engine output is greater than a preselected amount, operating the engine in a first mode with all the combustion chambers combusting a mixture of air and fuel, and when the desired engine output is less than a predetermined amount, transitioning to a second mode with a first number of the combustion chambers inducting air with substantially no fuel injected, and a second number of the combustion chambers combusting a mixture of air and fuel at an output per chamber greater than output per chamber when operating in the first mode before the transition, and engine output is less than when operating in the first mode before the transition.
By operating some of the cylinders with only inducted air, and combusting an air/fuel mixture in the other combustion chambers when desired engine output falls below a predetermined amount, the combustion chamber torque of the operating combustion chamber is increased while the overall engine torque falls. In this way, since the combusting chambers are operating at higher loads, they will have a higher combustion stability limit than heretofore possible. This higher limit may then be used to operate the combusting combustion chambers leaner than heretofore possible, resulting in still added fuel economy benefits.
Further, because the combusting chambers are operating at a higher load, greater engine efficiency is achieved than heretofore possible. And, the combination of operating the engine at higher load and leaner air/fuel ratios results in operation closer to wide-open throttle thereby reducing engine pumping losses and increasing efficiency.
Another advantage is that any of the combustion chambers may be selected as non-combusting chambers. Thus, the engine is not limited to disabling an entire engine bank as was the case with the prior approaches. One advantage is that chambers may be disabled in accordance with firing order resulting in smoother engine operation. Usage at idle, for example, is now practical.
Also note that there are various methods for injecting fuel into the engine combustion chambers. For example, the engine can be a direct injection engine where a fuel injector provides fuel directly into the combustion chamber. Alternatively, the engine can be of an indirect injection type where fuel injectors provide fuels into intake ports of an engine intake manifold coupled to the combustion chambers. Also, fuel can be in either a liquid or vapor, or combined, form. Also, when operating a combustion chamber group at an average air/fuel ratio, this can mean that there is some variation between the actual air/fuel ratio combustion in the various combustion chambers. Also, this average air/fuel ratio can be varied according to a desired air/fuel ratio. Also note that inducting air into the combustion chambers can include inducting air and fuel (liquid or vapor).
It is further noted that the desired engine output may comprise engine torque, engine power, engine acceleration, or similar measures of output.
The desired engine output may be provided in response to an operator command such as pedal position, a command from a vehicle speed control system, or command from a vehicle traction control system. The desired engine output may be determined from engine torque, engine power, or engine acceleration.
Feedback control also is provided to maintain actual engine output at a desired output.
In another aspect of the invention, the method comprises: determining a desired engine output; operating in a first mode by combusting an air/fuel mixture in all the combustion chambers at substantially a first air/fuel ratio when the desired engine output is greater than a preselected amount; when the desired engine output is less than a predetermined amount, operating in a second mode by inducting air with substantially no fuel injected into a first set of combustion chambers, and combusting an air/fuel mixture at substantially a second air/fuel ratio in a second set of combustion chambers at an output per chamber greater than output per chamber when operating in the first modes; and adjusting the air/fuel mixture in the second set of combustion chambers to achieve substantially the desired engine output when operating in the second mode.
When the second air/fuel ratio is lean of stoichiometry, the adjusting step comprises adjusting fuel injected into the combustion chambers. On the other hand, when the second air/fuel ratio is stoichiometric, the adjusting step may comprise adjusting air injected into the combustion chambers. Thus, a very versatile control system is achieved having rapid response time and accurate control. By enabling the first or second air/fuel ratios to be either lean, stoichiometric, or even rich, a wide range of authority is achieved to optimize engine efficiency over a wider operating range than heretofore possible.